The present invention relates to the general field of holographic storage systems and methods. More specifically the invention relates to a system and method for reflective holographic storage with associated multiplexing techniques.
General holographic storage systems are discussed in xe2x80x9cHolographic Memoriesxe2x80x9d, by Demetri Psaltis et. al., Scientific American, November 1995, which is hereby incorporated by reference. Holography is also discussed in the text Holographic Data Storage, by H. J. Coufal, D. Psaltis, and G. T. Sincerbox, Eds., copyright 2000, Springer-Verlag which is hereby incorporated by reference. The basic principles of holography involve the recording of an interference pattern formed between two beams of light, referred to as an object beam and a reference beam. The object beam is encoded with data in a two dimensional pattern. The reference beam is used to form the interference pattern with the encoded object beam and is subsequently used to reconstruct the data by illuminating the recorded pattern.
In a volume holographic storage medium, a large number of holograms can be stored in the same volume region using multiplexing techniques. There are several techniques for multiplexing holograms, including shift multiplexing, angle multiplexing, wavelength multiplexing, correlation multiplexing and phase multiplexing. Volume holography uses a thick recording medium, where the thickness dimension is associated with Bragg selectivity in the movement of the holographic storage medium in shift multiplexing or the angle change in angle multiplexing.
Shift multiplexing is a volume holography method for storing a plurality of images within a single holographic medium. Such shift multiplexing is discussed in xe2x80x9cShift Multiplexing with Spherical Reference Wavesxe2x80x9d, pages 2403-2417, by George Barbastathis et al, Applied Optics, Vol. 35, No. 14, May 10, 1996. Shift multiplexing generally involves the high density packing of successive holograms in an x-y array. Overlapping holograms produced by shifting the medium in the grating direction are differentiated by first-order Bragg selectivity.
FIG. 7 illustrates the basic setup of a typical prior art holographic system. The holographic storage system 700 includes a laser light source 710. The coherent light from the laser light source 710 is directed to a beam splitter 715, such as a polarizing beam splitter cube, which splits the light from laser light source 710 into a reference beam 720 and an object beam 725. Reference beam 720 is reflected by a turning mirror 730 to a lens 735. Object beam 725 is directed to a turning mirror 745 which directs the object beam to a Spatial Pattern Encoder 755, which encodes the object beam with data (an image). The object beam is then directed to a holographic storage media 750 with lens 780. Pattern encoder 755 may be a spatial light modulator (xe2x80x9cSLMxe2x80x9d), or any device capable of encoding the object beam, such as a fixed mask, or other page composer. The encoded object beam 725 is then directed to lens 780 that focuses the encoded object beam 725 to a particular site on the holographic storage media 750. Successive overlapping holograms may be recorded in a shift multiplex system by translating the holographic storage media 750 in a shift multiplex direction 788.
During readout of holograms previously stored in the holographic storage media 750, object beam 725 is blocked from transmission and a reference beam is projected at the same angle to the same spot on the holographic storage medium on which the desired information was previously stored. Diffraction of the reference beam with the previously stored hologram generates a reconstruction beam 782 that reconstructs the previously stored hologram. The reconstructed beam is transmitted towards imaging lens 784 that directs and images the reconstruction beam onto the plane of the optical detector 786. Optical detector 786 may be a conventional photodiode array, charge coupled device or other suitable detector array that transforms the encoded page into digitized data. In the prior art holographic storage system 700, spatial light modulator 755 and detector 786 are on opposite sides of holographic storage media 750. Lens 780 and lens 784 are also on opposite sides of holographic storage media 750, and are required to image the encoded object beam 725 onto the holographic storage media 750 and image the reconstruction beam 782 onto the detector 786, respectively. Lens 735 is required to image the reference beam 735 onto the holographic storage media 750.
Another prior art holographic system is described in xe2x80x9cHolographic 3-D Disk using In-line Face-to-Face Recordingxe2x80x9d, by Kimihiro Saito and Hideyoshi Horimai. The system described utilizes a photosensitive layer with a reflecting unit underneath. A reference beam passes through a first region of the media downward and a second region upwards. The direction of the information beam is opposite to that of the reference beam. Intersection between the reference beam and information beam results in a reflection type hologram. Shift multiplexing can be utilized for multiple recording.
Angle multiplexing is a volume holography method for storing a plurality of images within a single photorefractive medium. Such angle multiplexing is discussed, for example, in xe2x80x9cHolographic Memoriesxe2x80x9d, by Demetri Psaltis et. al., Scientific American, November 1995, and by P. J. van Heerden in, xe2x80x9cTheory of Optical Information Storage In Solids,xe2x80x9d Applied Optics, Vol. 2, No. 4, page 393 (1963). A typical system employing angle mutiplexing described in Holographic Data Storage, by H. J. Coufal, D. Psaltis, and G. T. Sincerbox, Eds., pages 343-397, copyright 2000, Springer-Verlag. Angle multiplexing generally involves storage of multiple pages of data in the same photorecording medium by altering the angle of the reference beam entering the media during storage of each page while maintaining the position of the object beam. Each hologram is stored in the same volume and is differentiated by Bragg selectivity. Bragg selectivity during angle multiplexing is described in Holographic Data Storage, pages 30-38 by H. J. Coufal, D. Psaltis, and G. T. Sincerbox, Eds., copyright 2000, Springer-Verlag. Any of the recorded holograms can be viewed by illuminating the photorecording medium with a reference beam set at the appropriate angle.
FIG. 8 illustrates a prior art system geometry in which the encoded object beam and the recording reference beam are counterpropagating. Such a system is described in xe2x80x9cVolume Holographic Multiplexing Methodsxe2x80x9d, by G. Barbastathis and D. Psaltis, published in Holographic Data Storage, pages 22-59, by H. J. Coufal, D. Psaltis, and G. T. Sincerbox, Eds., copyright 2000, Springer-Verlag, which is expressly incorporated herein by reference. This geometry is often preferred in wavelength multiplexed systems because it maximizes the optical wavelength Bragg selectivity. However, the prior art system requires that the object beam optics 810 and reference beam optics 815 be on different sides of the holographic storage media 820 in order for the beams to be counterpropagating. Thus, the system is not of a compact design since components are required on both sides of the holographic storage media 820.
Although the prior art systems offer the ability to store a large number of holograms within a holographic storage media, there are disadvantages to existing systems. Although providing for storing of multiple overlapping images, shift multiplexing requires a relatively thick recording medium. However, as the thickness of the photopolymer increases, recording of holograms is made difficult both by the absorption of light by the photosensitizer, and by the low viscosity of the photopolymer before exposure. Recording thick polymer holograms is discussed in Holographic Data Storage, pages 172-208, by H. J. Coufal, D. Psaltis, and G. T. Sincerbox, Eds., copyright 2000, Springer-Verlag. In addition, as the recording medium thickness decreases, the further the medium must be shifted prior to recording a successive hologram, reducing hologram storage density. Angular selectivity of the media during angle multiplexing also improves with recording medium thickness. Furthermore, the optics required by prior art systems require several lenses, and require components on both sides of the holographic storage media.
Thus, there has been a need for improvements in the storage of holograms. More specifically, there has been a need for more efficient hologram storage. In addition, there has been a need for more compact and less complex optics systems.
The present invention provides a solution to the needs described above through a system and method for reflective holographic storage with associated multiplexing techniques.
In a first embodiment of the invention, a method and system of recording successive holograms in a recording medium is presented. The method utilizes a multilayer holographic storage media comprising a reflective substrate layer, a polarization shifting layer disposed above the reflective substrate layer, and a photorecording medium layer disposed above the polarization shifting layer. A reference beam and an encoded object beam are propagated at a first direction to a first area of the photorecording medium layer, where the reference beam and encoded object beam have a same first polarization and interfere to produce a first interference grating. The reference beam and the encoded object beam are altered by the polarization shifting layer and then reflected with the reflective substrate layer to produce a reflected reference beam and reflected encoded object beam incident the photorecording medium at a second direction, where the reference beam polarization and a encoded object beam polarization are again altered with the polarization shifting layer so that the reflected reference beam and reflected encoded object beam have a same second polarization due to passing through the shifting layer twice. The reflected reference beam and reflected encoded object beam interfere to produce a second interference grating, with the first polarization and second polarization being different. Successive holograms are recorded by translating the multilayer holographic storage media or reference and object beam along a shift multiplex direction, where the reference beam and object beam are propagated to successive areas of the photorecording medium.
During reading out of a previously recorded hologram, a probe beam is propagated at an incident direction to the first area of the holographic storage media along the same path as the record reference beam. The probe beam is reflected by the reflective substrate to produce a reconstruction beam. The reconstruction beam is directed away from the holographic storage media along the same propagation path as the record object beam to a polarizing beam splitter and deflected to a detector. A further embodiment of the invention utilizes a phase conjugate probe beam to reconstruct a previously recorded hologram.
A further embodiment of the invention presents a method and system of recording successive holograms in a recording medium. The method utilizes a multilayer holographic storage media comprising a reflective substrate layer, a polarization shifting layer disposed above the reflective substrate layer, and a photorecording medium layer disposed above the polarization shifting layer. A reference beam with a first polarization is propagated at an incident direction to a first area of the photorecording medium layer. The reference beam is reflected by the reflective substrate and the polarization shifting layer introduces a polarization shift of the incident and reflected reference beam. A data encoded object beam with a second polarization is propagated at an incident direction to a first area of the photorecording medium layer. The object beam is reflected by the reflective substrate and the polarization shifting layer introduces a polarization shift between the incident and reflected object beam. The incident reference beam and reflected object beam interfere to produce a first interference grating, and the reflected reference beam and incident object beam interfere to produce a second interference grating. Successive holograms are recorded by translating the multilayer holographic storage media or reference and object beam along a shift multiplex direction, where the reference beam and object beam are propagated to successive areas of the photorecording medium.
An embodiment of the invention presents a further method and system of recording successive holograms in a recording medium. The method utilizes a multilayer holographic storage media comprising a reflective substrate layer, a polarization shifting layer disposed above the reflective substrate layer, and a photorecording medium layer disposed above the polarization shifting layer. The method also utilizes a wave plate ring with a hollow interior. A reference beam is propagated at an incident normal direction through the waveplate ring and through an imaging lens to a first area of the photorecording medium layer, and is reflected by the reflective substrate. A data encoded object beam is propagated from a spatial light modulator through the hollow interior of the waveplate ring at an incident normal direction through the imaging lens to the first area of the photorecording medium layer, and is reflected by the reflective substrate. The object beam and reference beam are reflected in a reflected direction through the photorecording medium layer and transparent substrate. The incident reference beam and incident object beam interfere to produce a first interference grating and the reflected reference beam and reflected object beam interfere to produce a second interference grating, and wherein the first polarization and second polarization are orthogonal so that there is no interference grating between the incident object beam and reflected reference or the incident reference and reflected object beam. Successive holograms are recorded by translating the multilayer holographic storage media or reference and object beam along a shift multiplex direction, wherein the reference beam and object beam are propagated to successive areas of the photorecording medium.
An embodiment of the invention presents a further method of recording a plurality of holograms in a recording medium. The method utilizes a multilayer holographic storage media comprising a reflective substrate layer, a polarization shifting layer disposed above the reflective substrate layer, and a photorecording medium layer disposed above the polarization shifting layer. A reference beam and an encoded object beam are propagated at a first direction to a select area of the photorecording medium layer, wherein the reference beam and encoded object beam have a same first polarization and interfere to produce a first interference grating. The reference beam and the encoded object beam are reflected with the reflective substrate layer to produce a reflected reference beam and reflected encoded object beam incident the photorecording medium at a second direction. The reference beam polarization and a encoded object beam polarization are altered with the polarization shifting layer so that the reflected reference beam and reflected encoded object beam have a same second polarization, wherein the reflected reference beam and the reflected encoded object beam interfere to produce a second interference grating, and wherein the first polarization and second polarization are orthogonal so that there is no interference grating between the incident object beam and reflected reference or the incident reference and reflected object beam. Subsequent holograms are recorded at the select area by varying the incident angle of the reference beam to the select area of the photorecording medium layer.
An embodiment of the invention presents a further method of recording a plurality of holograms in a recording medium. The method utilizes a multilayer holographic storage media comprising a photorecording medium layer disposed above a reflective substrate layer. A reference beam with a first polarization is propagated at an incident direction and incident angle to a select area of the photorecording medium layer, wherein the reference beam is reflected by the reflective substrate. A data encoded object beam with a second polarization is propagated at an incident direction to a select area of the photorecording medium layer, wherein the object beam is reflected by the reflective substrate. The incident reference beam and reflected object beam interfere to produce a first interference grating and the reflected reference beam and incident object beam interfere to produce a second interference grating. Subsequent holograms are recorded at the select area by varying the incident angle of the reference beam to the select area of the photorecording medium layer.
A further embodiment of the invention presents a system for recording and reading out holograms in a recording medium. The system utilizes a multilayer reflective holographic storage media comprising a reflective substrate layer, a polarization shifting layer disposed above the reflective substrate layer, and a photorecording medium layer disposed above the polarization shifting layer. The system further comprises a laser light source for providing a reference beam and an object beam, a rotatable beam deflector for varying the angle of incidence of the reference beam on the photorecording medium layer, a pattern encoder for encoding data on the object beam to produce an encoded object beam, wherein the reference beam and object beam are propagated to the photorecording medium layer during a hologram recording process using associated reference beam and object beam optics, and a detector for receiving a reconstruction beam during a hologram readout process.